The present invention relates in general to wrapping machines and has been developed with particular attention to its possible use in wrapping machines of the type normally known as "flow-pack" or "FFS" (form-fill-seal) machines.
In these machines, which are widely known in the art, the products to be wrapped advance in a substantially continuous flow towards a station or unit in which the wrapper is formed. A wrapping film constituted, for example, by laminated composites of paper-aluminium, various plastics materials, etc. is simultaneously advanced (from above or below) towards the station. In the wrapper-forming station, the film is wrapped around the products so as to form a generally tubular unfinished wrapper with a lower seal. In a closure station downstream of the wrapper-forming station, a pair of contrarotating jaws close the wrapper horizontally in the regions between successive products, separating the individual packages thus formed by a cutting action.
In many applications, the wrapping film is printed with inscriptions, graphic symbols, etc. which must appear in precisely determined positions relative to the product in the wrapper. Very tight tolerances are often imposed in this connection, for example, tolerances of the order of 1% of the overall length of the package.
There is thus a need to synchronise (or to "phase") the supply of the wrapping film precisely with the advance of the products and, in particular, with the action of the jaws which close the individual packages.
It has already been proposed in the art to use for this purpose optical means which can detect the positions of certain reference marks (for example, bands of a color which contrasts with the rest of the wrapper) reproduced on the film at given intervals correlated to the lengths of the products
In some solutions adopted in the art, the optical means are located in correspondence with the reel from which the wrapping film is drawn for supply to the wrapper-forming unit.
This solution has not been found completely satisfactory, however, since it does not allow account to be taken of any variations from the expected value of the intervals at which the graphic representations are reproduced on the film. In fact, for various reasons, these marks, which are usually printed on the film, may be spaced at slightly different intervals from one reel to another, or even on the same reel for reasons connected with the printing process. Further slight variations may also be induced in some films by variations in ambient conditions or by various longitudinal stresses to which the reel is subject during printing.
According to these known solutions, therefore, the positions of the reference marks are detected a long way upstream of the position where the jaws close the wrapper. In some cases, the distance between the unwinding reel and the jaws for closing the wrapper may even be some tens of times the length of an individual package (the periodic interval) in the direction of advance of the film. In practice, this means that the synchronisation carried out with reference to the position at which the reel is unwound may be wholly ineffective or erroneous since it is subject to an error equal to the variation of the periodic interval from its expected nominal value multiplied by the number of packages separating the optical reader positioned in correspondence with the reel from the region where the jaws close the wrapper.
At least in principle, this problem can be eliminated if the positions of the reference marks are detected immediately upstream of the region where the jaws close the wrapper. In practice, this means that the optical detection means are placed downstream of the wrapper-forming station so that they monitor not the flat film unwinding from the reel but the wrapper which has been wrapped around the articles to form a tube. Detection in this position, however, is extremely difficult; in fact, the surface of the wrapper to which the marks are applied is not kept rigorously at a constant distance from the optical detector. Indeed, this distance may vary to a considerable extent, for example, when (for various reasons) there is an interruption in the supply of the articles to the wrappers. In these circumstances, the tubular wrapper, which is usually kept extended by the products within it, tends to assume an almost circular cross-section (like an onion, so to speak) which adversely affects the accuracy of the detection.
Another disadvantage is the fact that, in many cases, the wrapping film has other markings (for example, writing, symbols, etc.) to which the optical detection means are sensitive, in addition to the reference marks used to generate signals for synchronising the machine; there is thus a risk that these other markings may interfere to a greater or lesser extent with the synchronisation.
At least in some cases, this problem could be eliminated by an appropriate selection of the position of the optical detection unit so that, for example, it is exposed exclusively to the passage of the optical reference marks and not to the other visible markings. For example, for packages which are intended to assume generally flattened shapes with the other markings reproduced exclusively on the upper face of the package, the detection unit could be positioned so as to monitor only one side of the package.
This solution is quite difficult to carry out, however since it makes the system even more vulnerable to any variations in the shape of the package. In fact, it is generally considered preferable to be able to carry out the detection substantially on the center line of the film.
The object of the present invention is to provide means which can resolve the problems described above and enable correct detection and synchronisation of the wrapping machine even at very high rates of operation (for example, 1000-1200 articles/minute).
According to the present invention, this object is achieved by virtue of a method and a wrapping machine having the characteristics recited specifically in the following claims.
One aspect of the present invention also relates to a method and a machine in which the advance of the wrapping film is synchronised with particular precision in dependence on the signal supplied by the optical detection means, by the generation of a location signal (preferably produced by a differential encoder driven by the film) which relates directly to the advance of the wrapping film and not, as is the case with most prior-art solutions, to the movement of the members from which the film is unwound.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.